With the constantly improving quality of color photocopies and printings and in an attempt to protect security documents such as banknotes, value documents or cards, transportation tickets or cards, tax banderols, and product labels against counterfeiting, falsifying or illegal reproduction, it has been the conventional practice to incorporate various security elements in these documents. Typical examples of security elements include security threads or stripes, windows, fibers, planchettes, foils, decals, holograms, watermarks, security inks comprising optically variable pigments, magnetic or magnetizable thin film interference pigments, interference-coated particles, thermochromic pigments, photochromic pigments, luminescent, infrared-absorbing, ultraviolet-absorbing or magnetic compounds.
Security threads embedded in the substrate are known to those skilled in the art as an efficient way for the protection of security documents and banknotes against imitation. Reference is made to U.S. Pat. No. 0,964,014; U.S. Pat. No. 4,652,015; U.S. Pat. No. 5,068,008; U.S. Pat. No. 5,324,079; WO 90/08367 A1; WO 92/11142 A1; WO 96/04143 A1; WO 96/39685 A1; WO 98/19866 A1; EP 0 021 350 A1; EP 0 185 396 A2; EP 0 303 725 A1; EP 0 319 157 A2; EP 0 518 740 A1; EP 0 608 078 A1; and EP 1 498 545 A1 as well as the references cited therein. A security thread is a metal- or plastic-filament, which is incorporated during the manufacturing process into the substrate serving for printing security documents or banknotes. Security threads or stripes carry particular security elements, serving for the public- and/or machine-authentication of the security document, in particular for banknotes. Suitable security elements for such purpose include without limitation metallizations, optically variable compounds, luminescent compounds, micro-texts and magnetic features.
With the aim of protecting value documents such as banknotes from being forged, optically variable security threads or stripe exhibiting color shift or color change upon variation of the angle of observation have been proposed as security features to be incorporated into or onto said value documents. The protection from forgery is based on the variable color effect that optically variable security elements convey to the viewer in dependence on the viewing angle or direction.
US 2007/0241553 discloses security elements for securing valuable articles having an optically variable layer that imparts different color impressions at different viewing angles and, in a covering area, a semi-transparent ink layer disposed on top of the optically variable, the color impression of the optically variable layer being coordinated with the color impression of the semi-transparent ink layer in the covering area when viewed under predefined viewing conditions.
WO 2007/042865 A1 discloses security elements comprising at least two contiguous areas having an identical or different optically variable coloring. The disclosed security element further comprises a single graphic marking which crosses with continuity the two areas having variable coloring so that the graphic marking straddles the two areas and is perfectly aligned.
EP 2 465 701 A2 discloses security elements for securing valuable articles comprising a stack layer made of an optically variable layer that conveys different color impressions at different viewing angles, a first portion with a first color-constant impression and a second color-constant impression and an individualizing marking. The optically variable layer and the two portions exhibiting two color-constant impressions are stacked in a covering region. The disclosed different layers are coordinated so that the color impression of the optically variable layer matches at a predetermined first viewing angle the color impression of the first portion and that the color impression of the optically variable layer matches at a predetermined second viewing angle being different from the first viewing angle the color impression of the second portion.
Alternatively or in addition to the protection against counterfeit or illegal reproduction obtained by the optically variable properties described hereabove, security threads or stripes comprising holographic structures have been developed. Commonly used processes for producing such optically variable threads comprising an holographic structure consist of laminating a partially de-metalized hologram layer on top of a fully coated color-shifting layer; such lamination leading to highly thick security threads which may cause difficulties during the integration of said threads in paper.
WO 2004/048120 A1 discloses security elements comprising at least two adjacent regions, wherein one of the regions is an optically variable and the other region has a layer of material with constant reflection. The disclosed security element comprises regions forming areas without material in order to form graphic makings, characters and the like that can be detected visually. The disclosed optically variable layer of material may be constituted by holographic material such as for example a holographic lacquer over which an embossing is performed in order to impress a holographic image.
U.S. Pat. No. 8,534,710 discloses security threads comprising a stack layer made of an optically variable layer that conveys different color impressions at different viewing angles, and a color-constant layer comprising an ink layer and a metal layer. The optically variable layer and the color-constant layer are stacked in a covering region, while at most one of the optically variable layer and the color-constant layer is present outside the covering region. The color impression of the stacked layers in the covering region and the color impression of the one layer outside the covering region are matched with each other when viewed at a predetermined viewing angle. It is further disclosed that a diffractive embossing pattern may be embossed in the optically variable layer so as to realize so-called colorshifting holograms, for example, in which the colorshifting effect of the optically variable layer is combined with a holographic effect.
U.S. Pat. No. 8,381,988 discloses security threads comprising a first and a second layer of a colourshifting material at least partially overlying each other and each having different colourshifting properties and, at least partially applied over an exposed surface of one of the colourshifting layers, a light control layer which may be a microprismatic film prepared by coating the colourshifting layer with a thermoplastic embossing lacquer and then using an embossing tool to create the light control structure with the application of heat and pressure.
US 2011/0012337 discloses security threads in which a) a colorshifting thin-film element in the form of an absorber layer, a dielectric layer and reflection layer and b) a relief pattern present in an embossing lacquer layer are stacked. The disclosed embossing lacquer layer having the relief pattern is metalized only in sub-regions so that the colorshifting thin-film element is visible when observed from the side of the relief pattern through the non-metal metalized sub-regions. However, the disclosed security threads comprising a non-printed colorshifting thin-film element may suffer from a low flexibility in terms of design and color combinations.
A need remains for providing sophisticated security threads or stripes combining high visual attractiveness with a highly sophisticated design so as to further increase the resistance against counterfeiting or illegal reproduction of security documents comprising said security threads or stripes.